This invention relates to a digital data transmitting device for use in a digital data communication network.
Such a digital data transmitting device is used in the network as a first digital data transmitting device together with a first digital data receiving device. The network comprises second digital data transmitting and receiving devices at a remote site or place and first and second digital communication paths or channels which are connected to the first digital data transmitting and receiving devices and to the second digital data transmitting and receiving devices, respectively.
Each digital communication path has either a restricted or an unrestricted transmission characteristic. More particularly, a bit sequence of digital data should always satisfy a certain condition for reliable transmission through a digital communication path of a restricted transmission characteristic. For example, the condition is such that fifteen or more binary zeroes should not continuously appear in the bit sequence and that three or more binary ones should occur in each partial sequence of twenty-four bits. On the other hand, the bit sequence need not satisfy any condition at all for a digital communication path of an unrestricted transmission characteristic.
A general bit sequence may have a part satisfying such a condition and another part which does not satisfy the condition. On transmitting the general bit sequence through a digital communication path of a restricted transmission characteristic, it is necessary for reliable transmission that the condition should be satisfied throughout the general bit sequence. For this purpose, stuffing is carried out for the general bit sequence. In other words, each of the above-mentioned first and second digital data transmitting devices includes a stuffing circuit to which the general bit sequence is supplied as a device input bit sequence. Otherwise, it is impossible to transmit the general bit sequence reliably through the digital communication path of the restricted transmission characteristic.
In the manner which is described in U.S. Pat. No. 4,353,129 issued to Mitsuo Nishiwaki, assignor to NEC Corporation, and will later be described more in detail, the stuffing circuit monitors at first the device input bit sequence by dividing the bit sequence into blocks, each having a block duration. The stuffing circuit thereby detects necessity and unnecessity of carrying out the stuffing for the respective blocks. Whenever the necessity is found, the stuffing circuit carries out the stuffing by forcibly adding a binary one bit as a stuff bit to a unit sequence of n bits where n represents a predetermined natural number. For the above-exemplified condition, the natural number may be equal to seven. In this manner, the stuffing circuit modifies the device input bit sequence into a stuffed bit sequence for reliable transmission through the digital communication path of the restricted transmission characteristic.
Attention should be directed to the fact that (n+1) bits are necessary in the stuffed bit sequence for the unit sequence in each block for which the necessity of stuffing is detected. When both of the afore-mentioned first and second digital communication paths has the unrestricted transmission characteristic, it is possible to raise efficiency of transmission of the device input bit sequence by transmitting the input bit sequence intact as it is. Inclusion of the stuffing circuit is therefore undesirable for a digital communication path of the unrestricted transmission characteristic in that the device input bit sequence is transmitted at a reduced transmission rate, namely, only ineffectively transmitted, at least at a part to which the stuffing is carried out.